The present invention relates to the art of medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
The medical field has found MRI to be a valuable diagnostic tool for the non-invasive study or examination of a patient""s anatomy. Various MRI scanners and apparatus have been describe in detail in the prior art. As is known in the art, by applying a particular combination of radio frequency (RF) pulses and magnetic gradients to a spin system set up in a region of interest, a signal therefrom (often comprising a plurality of echos) can be induced, received and processed into an image representation of the region of interest.
Generally, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Nuclei in the subject have spins which in the presence of the main magnetic field produce a net magnetization. The nuclei of the spin system precess in the magnetic field at the Larmor frequency, i.e., the resonant frequency. Radio frequency (RF) magnetic fields at and/or near the resonant frequency are used to manipulate the net magnetization of the spin system. Among other things, RF magnetic fields at the resonant frequency are used to, at least partially, tip the net magnetization from alignment with the main magnetic field into a plane transverse thereto. This is known as excitation, and the excited spins produce a magnetic field, at the resonant frequency, that is in turn observed by a receiver system. Shaped RF pulses applied in conjunction with gradient magnetic fields are used to manipulate magnetization in selected regions of the subject and produce a magnetic resonance (MR) signal. The resultant MR signal may be further manipulated through additional RF and/or gradient field manipulations to produce a series of echos (i.e., an echo train) as the signal decays. The various echos making up the MRI signal are typically encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data line""s position in k-space (i.e., its relative k-space row) is typically determined by its gradient encoding. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space (or reciprocal space) data.
Patient motion can often be a factor in MRI. For example, patient motion can give rise to degraded image quality as a result of motion artifacts. Typical motions that may present issues include cyclical type motions resulting from natural biological functions, i.e., cardiac and respiratory motion. As is known in the art, however, the motion of body parts can be monitored in MRI by generating, collecting and processing navigator echos, which are separate from the echos used for imaging. The navigator echos or navigator signal is typically obtained as a one-dimensional (1D) projection of some region of the body that is moving in some cyclic manner, e.g., the chest wall, the diaphragm, cardiac blood flow, etc. As is known in the art, a motion measure or motion data is commonly derived from the resulting navigator data by a series of processing steps including, e.g., Fourier transformation, summation, integration, edge detection, cross-correlation, least squares error determination, linear phase shifting, etc.
The issue concerning the present invention is the manner in which the motion measure or motion data is employed or processed to gate the image data to the motion, i.e., to decide the acceptance, rejection and/or binning of image data or to trigger generation or regulate acquisition of the image data, preferably, to substantially reduce or eliminate motion artifacts in a reconstructed image representation.
In general, gating is alternately used prospectively to trigger the generation and/or regulate the acquisition of image data to repeatedly correspond to a desired location in a motion cycle, or it is used retrospectively to select or identify a specific fraction of image data from a set of continuously acquired image data, where the specific image data corresponds to a desired location in a motion cycle. In either case, a complete set of image data for a particular image reconstruction can be generated and/or acquired over a number of motion cycles and still be made to consistently correspond to the same location in the motion cycle. In this manner motion artifacts can be substantially reduced or eliminated insomuch as the reconstructed image is reconstructed from image data that all corresponds to the same location in the motion cycle.
In typical previously developed techniques, motion data is obtained from navigator echos generated via a prescan pulse sequence and used to observe motion patterns prior to an imaging scan. During the imaging scan then, navigator echos are periodically inserted, e.g., before or after lines or other segments of image data. The inserted navigator echos are processed and the resulting motion data used to determine the obtained image data""s relative correspondence to the previously observed motion pattern. The image data is then optionally gated (i.e., rejected, accepted, binned, triggered or acquired) accordingly.
The aforementioned previously developed techniques have a number of undesirable limitations. For example, the resulting navigator data derived from the inserted navigator echos, and hence the corresponding image data, is referenced and/or correlated to a fixed motion pattern observed prior to the imaging scan. Consequently, the technique does not and cannot compensate for dynamic changes in the motion experienced during the imaging scan, e.g., as may result from an irregular or changing breath volume. Moreover, while substantially stable, the motion pattern during the imaging scan may be different than the motion pattern observed in the pre-scan. Again, when a fixed pre-scan motion pattern is used as a point of reference, compensation cannot be achieved for a relative change in the motion pattern at the time of imaging. Additionally, the aforementioned techniques with their fixed frame of reference cannot compensate for random non-cyclical motions, e.g., as may result from singular voluntary (albeit perhaps unintentional) patient shifts or other like haphazard movements.
The present invention contemplates a new and improved technique for navigator echo analysis and motion gating in MRI which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of imaging a patient with an MRI scanner is provided. The method includes imaging a region of interest of the patient with the MRI scanner and acquiring image data resulting from the imaging. An image of the region of interest is reconstructed from the image data. The method also includes generating navigator echos during the imaging, collecting the navigator echos and deriving, from each navigator echo, a measurement of patient motion experience during the imaging. A historical record of the measurements is maintained. Parameters for the acquisition of image data are selected to compensate for motion, or specific image data is selected for reconstruction, based on the historical record.
In accordance with a more limited aspect of the present invention, the historical record is iteratively updated during the imaging as successive measurements are derived.
In accordance with a more limited aspect of the present invention, the historical record comprises all of the measurements obtained during the imaging.
In accordance with a more limited aspect of the present invention, the historical record comprises a fraction of all the measurements obtained during the imaging, said fraction corresponding to those measurements most recently obtained.
In accordance with a more limited aspect of the present invention, the method further includes sorting the measurements in the historical record by their respective values and determining a cumulative distribution from the sorted measurements such that the cumulative distribution acts as a reference for gating the image.
In accordance with a more limited aspect of the present invention, the method further includes associating the image data with measurements based on the respective image data""s temporal proximity to the navigator echo from which each measurement was derived and determining at least one of a maximum and a minimum measurement limit from the historical record such that image data associated with a measurement beyond the limit is not used in reconstructing the image.
In accordance with another aspect of the present invention, a medical imaging apparatus includes an MRI scanner which obtains images of a patient positioned therein. The MRI scanner is also operable to produce navigator echos during imaging of the patient with the MRI scanner. A motion detection means processes each of the navigator echos to obtain motion measurements representative of a state of motion of the patient during imaging, and a storage device maintains a historical record of the motion measurements. A gating processor accesses the historical record and gates the images obtained from the MRI scanner.
In accordance with a more limited aspect of the present invention, the medical imaging apparatus further includes a sorter which sorts the motion measurements in the historical record according to their respective values, and means for determining a cumulative distribution from the sorted motion measurements. The cumulative distribution then acts as a frame of reference for the gating carried out by the gating processor.
In accordance with a more limited aspect of the present invention, the medical imaging apparatus further includes a reconstruction processor which reconstructs image data obtained from the MRI scanner into images and a statistical analyzer which determines statistical properties of the cumulative distribution. The statistical properties are used to compute at least one of a maximum and minimum motion measurement limit such that image data obtained from the MRI scanner when an associated motion measurement is beyond the limit is not used in reconstructing the image.
In accordance with a more limited aspect of the present invention, the historical record is iteratively updated during the imaging as successive motion measurements are obtained.
In accordance with a more limited aspect of the present invention, the historical record comprises all of the motion measurements obtained during the imaging.
In accordance with a more limited aspect of the present invention, the historical record comprises a fraction of all the motion measurements obtained during the imaging, said fraction corresponding to those motion measurements most recently obtained.
In accordance with a more limited aspect of the present invention, the medical imaging apparatus further includes a human-viewable output device for rendering images obtained from the MRI scanner.
One advantage of the present invention is that it functions for not only regular cyclical movement but for irregular or unstable movements also. The technique of the present invention is fairly model free, i.e., it make no assumptions about periodicity, frequency, etc.
Another advantage of the present invention is that the technique compensates for dynamic changes in the motion experienced during the imaging experiment.
Another advantage of the present invention is that it can be applied to motion measurements from multiple navigators, and cumulative distributions in multiple dimensions may be calculated and correlated.
Yet another advantage of the present invention is that spurious image data may be rejected when associated the motion measurements are outside maximum and minimum limits.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.